Unmanned Systems Technology 012 | AutoNaut USV | Connectors | Unmanned Ground Vehicles | Cobra Aero A33i | Intel Falcon 8+ UAV | Propellers | CES Show report

38 Focus | Connectors types of pins, from single-ended and differential to power. Here the design of the contact is a critical differentiator. Commercial connectors have contacts with a tuning fork design and a single point of contact, while high-reliability connectors will have multiple points of contact. These are more complex to build and test but provide much higher levels of reliability. System designers will mix high- reliability and commercial connectors together, for example using the former for power lines and comms systems, which are not mission-critical, and the latter for systems such as air temperature and pressure sensors. These can be qualified to military standards or use the same designs without the higher levels of testing to reduce the costs. A low-profile, dual-row interconnect for stacking and cable mating designed for areas where PCB space is at a premium uses a four-finger patented contact design and is built from a single piece of metal. The four fingers maintain electrical contact through high vibration and shock, as any movement forces a better connect on one of the fingers. Contacts with more points of contact are being developed, but four gives good resistance to vibration and bump shock, with vibration resistance of 20 g , acceleration resistance of 50 g , 100 g for shock and 40 g for bumps. There is currently patent work being carried out on contacts with more points of contact. A single piece of metal means fewer stages in manufacturing with less opportunity for debris and so higher reliability, but at a higher cost per connector. Milling a contact from a single block of alloy is more expensive in time and materials than combining several elements that are pressed out of a sheet of metal, but the process of producing such single-block contacts is a closely guarded commercial secret. All of this is intended to ensure that the connector is not the point of failure for the system. Up to 50 contacts fit into a rectangular connector on a 2 mm pitch and can be a mix of signal and power lines with up to 2 A of current per contact and a temperature range of -65 to +150 C. The connectors are tested for 1000 hours at extreme temperatures to monitor any change in contact resistance, and beryllium copper is used as this gives the best temperature performance. There can be a lot of focus on pitch but at the end of the connector there is also a moulding and retention, so connector systems can reduce these elements to a minimum while still carrying 2 A per contact. That gives a 45% space saving and up to 75% weight saving over other high-performance connectors such as Micro-D while supporting 1000 mating cycles. The important role the pitch of the connector plays is shown by the specification of a 3 mm pitch product. This can provide up to 10 A in a three-, six and ten-way configuration, again operating from -65 C up to 175 C, with the current capability de-rating at the higher temperatures. The material used for the shell is also an important factor in the overall design, as it has to provide protection as well as strength. Engineering-grade polymer materials are often customised by the suppliers for individual connector makers, and material such as military-grade polyphenolsulphide provide for good temperature characteristics and low outgassing for space applications. Good practice is that the materials are halogen-free to avoid problems with regulations such as RoHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment Directive 2011/65/EU) that apply in Europe. This does not apply for many UAV systems but may do in the future, and using these newer materials avoids future redesigns. For the solder joints, the main issue is shelf life for the connectors, which is why crimping is still a popular option. Using tin solder has a risk of tin whiskers forming over time that can cause shorts, which is why tin/lead has been used in the past but does not meet the RoHS requirements because of the lead content. Coplanarity As connectors get smaller, coplanarity – or the ‘flatness’ of the connector – becomes more of an issue when connected to a board. Many connectors still use through-hole or crimping to ensure they are securely attached, but that is increasingly unpopular with mainstream board manufacturing processes. That is driving the move to connectors that can be used with standard surface mount processes, but these have to survive the high- temperature ovens that melt the solder February/March 2017 | Unmanned Systems Technology Gecko connectors provide a low-profile, dual-row interconnect solution for stacking and cable mating where PCB area is at a premium. (Courtesy of Harwin)